The invention relates to the transmission of high speed data signals in a multi-hop network and relates in particular to a method for minimizing reflection effects on data received by a receiver.
In a data communication network, data signals originating on a driver circuit board or card at a source typically reach a board or card in a destination or receiving system only after traversing a series of connections through intermediate circuit boards or transmission media.
A problem which must be solved in such networks is how to assure a high signal integrity at the receiver input. A part of the solution is to suppress as much as possible the effects of signal reflections occurring at connections on the signal path between the driver output and the receiver input.
For critical high speed systems, a partial solution can be accomplished for point to point topologies by adding a resistance at the output of the driver such that the total output resistance is equal to the characteristic impedance of the transmission media and also adding a terminating resistance at the input of the receiver such that the total impedance of the receiver is equal to the characteristic impedance. Unfortunately, even the above implementation does not completely eliminate signal reflections which degrade signal integrity at the receiver input. First, while it is possible to add resistance at components in an effort to achieve a characteristic impedance having a fixed nominal value, the actual characteristic impedance of a component may or may not be equal to the nominal characteristic impedance due to process tolerances. Assuming a nominal characteristic impedance of 50 xcexa9 a circuit card with a tolerance of 15% may have an actual impedance as high as 57 xcexa9 or as low as 42 xcexa9. Similar variations may exist for transmission media interconnecting such cards. Even though all of the components on the signal path may have the same nominal characteristic impedance, mismatches in the actual characteristic impedances can still result in signal reflections.
A possible solution to the above problem is to use only coaxial cables to connect a driver card to a receiver card since the actual characteristic impedance of such cables can be controlled to a tolerance of plus/minus 5% of nominal. This solution isn""t practical except for the simplest situations. In typical networks, where the number of connections may be as high as a thousand, it becomes impossible to use coaxial cable for all of them because such cables take too much room, cost too much and take too much time to install and connect.
Another source of reflections is parasitic capacitance at the interfaces between the components on the signal path. For data transmission rates greater than 500 Mbit/s, the presented impedance for a parasitic capacitance in the range of one picofarad is on the order of 10 xcexa9 which acts as a short-circuit for high frequencies.
The invention is a method and a system for maintaining signal integrity by enabling signal perturbations resulting from reflections in a connection between a driver and a receiver to occur at times other than the times of transitions in a received data signal.
The invention relates to a method for adjusting the signal transmission delay in a data transmission system wherein a driver transmits high speed data to a receiver through a plurality N of transmission media connected together, the link between the driver and the receiver being composed of a plurality of N elements, the length of the element located on each transmission medium xe2x80x9cixe2x80x9d being Li with xe2x80x9cixe2x80x9d being an integer comprised between 1 and N. The method consists in introducing into each element in a transmission medium xe2x80x9cixe2x80x9d a delay xcex94Ti equal to:       Δ    ⁢          xe2x80x83        ⁢    Ti    =            Ki      ·              T0        2              -          Li      Vi      
with Ki being equal to       n    i        N    +    1  
modulo 1, and wherein T0 is the duration of a data pulse between its rising transition and its falling transition, ni is an integer equal to 1 or a number which is prime with N+1, and Vi being the signal propagation speed of data signals in the element.
According to a preferred embodiment of the invention, the added delay has a value xcex94Li equal to:       Δ    ⁢          xe2x80x83        ⁢    Li    =                    Vi        ·        Δ            ⁢              xe2x80x83            ⁢      Ti        =                  Vi        ·                              n            1                                N            +            1                          ·                  T0          2                    -      Li      
with Vi being the propagation speed of the data signals in the transmission medium xe2x80x9cixe2x80x9d.